U.S. Pat. No. 4,005,330 to Homer H. Glascock and John M. Anderson, and allowed U.S. patent application Ser. No. 642,142 filed Dec. 18, 1975 by John M. Anderson, now U.S. Pat. No. 4,017,764, describe induction ionized, electrodeless fluorescent lamps wherein a high frequency, solenoidal electric field is established by a transformer which is centrally disposed with respect to a substantially globular lamp envelope. The lamps described in those patents may be produced in a form which is electrically and mechanically compatible with common, screw-base, incandescent lamps and which provides substantially more efficient operation than conventional incandescent lamps.
The transformer which is utilized in the above-described fluorescent lamps generally comprises a primary winding coupled to a closed loop magnetic core, which is typically ferrite. The core is centrally disposed with respect to the lamp envelope and is coupled to a fill-gas therewithin. During lamp operation, power is transferred to a plasma in the gas which forms a "single turn secondary" linking the transformer core. The voltage drop around the plasma secondary, which is a function of the lamp geometry, the fill-gas composition, and the fill-gas pressure, acts to determine the peak magnetic flux level within the transformer core. Power dissipation within ferrite cores is known to increase, with a commensurate decrease in transformer efficiency, at increased magnetic flux levels. It is, therefore, desirable to reduce the plasma voltage drop and, thus the peak transformer magnetic flux level, in these lamps.
The saturation flux density in those ferrite materials which are suitable for use in the above-described transformer cores decreases rapidly if the core temperature exceeds a critical level, typically in the vicinity of 125.degree. C. It is, therefore, necessary to provide structures for limiting the maximum temperature of magnetic cores within induction ionized lamps. The above-referenced patents describe structures for transferring heat from a magnetic core to the external environment of a fluorescent lamp.
U.S. Pat. No. 4,005,330 and allowed U.S. patent application Ser. No. 642,142, now U.S. Pat. No. 4,017,764, are incorporated by reference in this specification, as background material.